Brief Description Of The Prior Art
In U.S. Pat. No. 4,796,676, I describe a fluid containment system for more safely containing fluids so as to minimize the risk of unwanted leakage of the contained fluid to the environment of the system. Thus, for example, the risk of aquifer contamination resulting from the percolation of leaked liquids is significantly decreased, as well as the economic loss which must be ascribed to such leakage.
The patented fluid containment system described includes a conventional metallic tank of the type used to contain hundreds or thousands of gallons, of, for example, gasoline or diesel liquid, most often in an underground location. Additionally, a special flexible liner or bladder (also called an internal tank) is fitted within the external rigid metallic tank. A vacuum is developed between the flexible internal tank and the external tank. Continuous monitoring of the vacuum provides an indication of the fluid integrity of both the rigid metallic external tank, and of the flexible liner. It is not possible to be certain that the rigid external metallic tank has not developed leakage, or will not soon do so. Moreover, once the external tank commences to leak and thus affords no significant containment redundancy, the external wall thereafter merely offers mechanical support to the internal tank or liner. Until expensive repair of the metal external tank is effected, it is difficult to effectively employ an interstitial vacuum monitoring system to afford an immediate and accurate indication of leakage development in the internal liner.
Systems of the type described, which rely basically upon a rigid external tank and a single interior liner are illustrated and described in a number of U.S. patents.
U.S. Pat. No. 4,408,628 to Robert J. Monck relates to a system and method for repair of leaking storage tanks containing ground water contaminant fluid. All of the tank openings originally used by appurtenances are plugged liquid tight, except for the tank vent system. A flexible fluid tight containment means is formed within the tank in the shape of the tank interior. A sampling means is installed between the outside of the containment means and low points of the tank interior for the detection and removal of ground water leaking into the tank, and for detecting any fluid leak through the containment means. The protective area covered by the sampling means is increased by the addition of corrugated spacers between the containment means and the lower third of the tank which is the most subject to leaks. The containment means is a flexible material which is inert to the contained liquid, and sufficiently pliable and flexible to permit it to be conformed to the interior shape of the external tank.
West German Patent 1,966,825 discloses a tank leakage detection system which accomplishes leak detection by monitoring the vacuum in a space between the walls of a double-skinned fluid containment tank. The vacuum is maintained by a motor driven exhauster controlled by an automatic switch. An automatic timer switch turns on the exhauster after an adjustable time interval.
An underground gasoline storage tank hanging in an expandable bladder-like primary tank position within the interior of a rigid secondary tank is disclosed in Strock U.S. Pat. No. 4,648,523. The interior primary tank is collapsible and is a flexible-expandable bladder located within the rigid secondary tank or shell. The bladder is desirably made of a durable, flexible fluid-impervious material, such as an elastomeric or rubber-like coated material which is impermeable and chemically inert to gasoline or other materials to be stored in the tank. The rigid external or secondary tank can typically be made of low-carbon steel or other rigid material such as fiberglass, is preferably coated with a rust inhibitor and is cathodically protected to minimize external corrosion. The containment system may have yet another intermediate protective barrier wall which protects and isolates conduits placed inside the tank system, and isolates these conduits from the expanded internal bladder.
A gasoline containment storage tank system which comprises a rigid inner tank encased by a flexible outer jacket. A leak detecting means is associated with the interstitial space between the inner tank and the jacket, all as disclosed in Sharp U.S. Pat. No. 4,607,522. Another embodiment of this invention disclosed in the Sharp patent is an outer rigid tank which contains a bladder within it, together with a leak detecting system located between the tank and the bladder. The space between the inner and outer tanks is filled with impervious material. In one embodiment of the invention, a vacuum pump is used to develop a negative pressure in the closed space between the inner bladder and the outer tank.
A slightly older patent to Sharp is U.S. Pat. No. 4,524,609. This patent discloses a gasoline storage tank system which comprises a rigid outer tank fitted with an internal bladder and with an analyzer. In one embodiment of the invention, the bladder itself has a flexible, double-wall. In this embodiment, the double-wall bladder located inside the rigid outer tank includes an inner bladder and an outer bladder. Optionally, some means is positioned between the two bladders to maintain a spaced relationship. The space between the two bladders is monitored for leakage of gasoline through the inner bladder. The patentee states that the advantage of the double bladder embodiment of the invention is that it provides another measure of protection against gasoline leakage, and can be considered a fail-safe design. The system is said to be quite useful where the rigid outer tank has earlier suffered structural damage. Monitoring of the system for leakage is said to be continuous.
Bruce R. Sharp U.S. Pat. No. 4,523,454 discloses a storage tank system for storing gasoline and comprising a rigid inner tank encased within a flexible outer jacket, with a leak detecting means associated with the closed space between the inner tank and the flexible outer jacket.
In Sharp U.S. Pat. No. 4,685,327, the storage tank system disclosed includes a rigid inner tank for containing gasoline. This tank has a sealed manhead with a dispensing line extending up through the manhead. A sleeve encompasses the manhead and a jacket encases the tank and at least a part of the sleeve. A detector is associated with the closed space between the inner tank and the jacket to detect leaks from the inner tank into the interstitial space between the inner tank and the surrounding jacket. The jacket is made of a flexible material, such as a synthetic polymer, or the jacket may be made of a rigid material such as steel or fiberglass.
In West German Patent 2,161,564, a leakage indicator system for a double-walled heating oil tank is disclosed. The system produces a vacuum in the interstitial space between the two walls of the storage tank system. The indicator system also contains a pressure-dependent switch arrangement to control the vacuum pump, and an alarm unit to monitor several containers.
West German Patent 2,352,544 discloses a double-walled container with a pump used to pressurize the space between the two walls. A switch is positioned to be activated by the pressure in the pressurized interstitial space, and the switch is connected to an alarm system.
In West German Patent 2,225,456, a leak indicator and testing device for gasoline tanks of the double-walled type is disclosed. The solid outer container and flexible inner container define an evacuated cavity between them, and the suction line from the vacuum pump used to develop the vacuum in the cavity issues into that cavity and goes to the bottom or sump thereof at the lowest level between the solid outer container and the flexible inner sleeve. The suction line from the vacuum pump runs down to the bottom of the cavity or sump through a slot formed down to this level within the inside wall of the solid outer tank.
A double-walled tank is provided for the storage of fuel oil or other liquids in Durkop U.S. Pat. No. 3,848,765. This system includes spaced inner and outer tanks. The inner tank can be made to be easily removable from, and insertable into, the outer tank. A pump is connected to a tube which extends into the space between the tanks to evacuate this space, and this vacuum is continuously monitored in order to detect leakage.
In Butts U.S. Pat. No. 4,568,925, an improved leak detection system for detecting leakage from underground storage containers and pipelines is disclosed. A leakage sensor is positioned adjacent the fuel storage container, and an impervious enclosure is used to extend around and enclose both the sensor and the container.
Another double-walled containment system patent is issued to Bruce R. Sharp, and is denominated U.S. Pat. No. 4,708,015. This system, like that disclosed in some of Sharp's other patents, includes a rigid inner tank which is encased by a flexible, yet chemically resistant outer jacket. This forms the closed space between the two elements. The system is monitored for leakage in a reliable and economic fashion. The space between the vessels is at least partially filled with a leak detecting liquid and a non-visual leak detection means is associated with this space for detecting any change in the level of that detecting liquid.
In yet another Sharp patent, U.S. Pat. No. 4,696,186, a fluid containment system particularly well suited for gasoline is disclosed. This system includes an outer rigid tank with an internal bladder. A leak detecting means is positioned between the external tank and the internal bladder. The bladder has openings into it which are sealed with the outer shell at a certain location, and gas impervious material is positioned between the bladder and the wall of the outer tank, and is securely adhered to each of these vessels.
In Mooney U.S. Pat. No. 4,651,893, an underground liquid storage tank assembly is disclosed, and this assembly includes an inner vessel which is preferably constructed of steel or other strong, relatively rigid material, and a seamless outer containment shell which is a corrosion resistant material, such as fiberglass. An intermediate spacer member is provided between the inner vessel and the outer containment shell to hold the two members rigidly together in spaced relation to each other, and to allow for free passage inside the outer containment shell of any liquid which might leak from the inner vessel. A structure is provided inside the outer containment shell to enable leaking liquid from the inner vessel to be detected.
Butts U.S. Pat. No. 4,672,366 discloses a system for containing petroleum products in a subterranean location which includes a steel or other rigid metallic tank positioned within an impervious enclosure surrounding the tank. A leak-sensing means is positioned at one end of the tank, but inside the impervious enclosure so that the leak sensor is actuated upon the occurrence of leakage from the tank. One type of material which can be used to make the liquid impervious enclosure is high density polyethylene.
West German Patent 2,735,804, issued in 1979, discloses a leakage detection and measuring system in a double-walled storage vessel. The internal storage vessel is for the primary containment of the liquid. A secondary or external storage vessel surrounds the internal storage vessel and a space is defined between the two. Pressure in this space is monitored for liquid penetration, and in doing so, the pressure is kept below atmospheric by a vacuum pump operated with a pressure and/or time dependent control. Separate evacuation and pressure measuring pipes are provided. When the pressure in the test space rises owing to unavoidable harmless leakages, the pressure is automatically maintained within a certain pressure range according to a predetermined program. Delay or failure to reach the pressure range lower threshold is taken as indicative of a leak. Means is provided which signals a pressure rise in the evacuation pipe above the lower pressure threshold.